Pressure test it.
But here's my nickle; Let her cool off.Pull the radcap off, top up the coolant, and warm her up. If it still behaves the same way, with zero psi in the cooling system, then I bet the head gasket let go. But I have never seen a gasket blow cylinder pressure in that direction. So that then points to a failed cooling system transfer. And that would most likely mean the intake gasket, most likely..
As others have suggested; it is much more likely that the intake is not pulling down properly; and that is relatively easy to prove.
As the heads move down they narrow the valley, forcing the intake higher. As the intake floats up, eventually the bolt holes don't line up. But if you manage to get them just barely in there, then there is no room for the bolts to draw the intake down. So the bolts tighten up reasonably fine cuz the torque spec is not that much, and so it feels Ok. But the fact is that it's not tight were it counts and out flows the water. The proof for this will be right on the gasket when you take it apart; there will be little or no clamp imprint on it or it will have a heavy imprint on perhaps the top, and no imprint on the bottom; or such like. That is your proof. That proof may already exist on the old gaskets if you still have them and if they were FelPros. Cheaper gaskets, or gaskets that were glued down, will usually/often be destroyed at teardown. I've had good success with dry FellPros.
The solution for this situation is not to elongate the holes. That mighta worked with a small misalignment, but IMO, your situation may be past that. Check out the previous gasket and the evidence may be right there on it.
